Showing foreign key attributes on Django Admin

admin.py

from django.contrib import admin
from django.core.urlresolvers import reverse
from .models import Carro, Fabrica, Pais


class CarroAdmin(admin.ModelAdmin):
    list_display = ('nome', 'fabrica', 'pais')

    def queryset(self):
        qs = super(CarroAdmin, self).queryset()
        return qs.select_related('fabrica__pais__codigo', 'fabrica__pais__id')

    def pais(self, obj):
        pais_edit_url = reverse(
            'admin:cars_pais_change',
            args=(obj.fabrica.pais.id,))

        return '<a href="{url}">{codigo}</a>'.format(
            url=pais_edit_url,
            codigo=obj.fabrica.pais.codigo)
    
    pais.allow_tags = True
    pais.short_description = 'Codigo do pais'
    pais.admin_order_field = 'fabrica__pais__codigo'

    class Meta:
        model = Carro

admin.site.register(Carro, CarroAdmin)
admin.site.register(Fabrica)
admin.site.register(Pais)

models.py

from django.db import models


class Pais(models.Model):
    nome = models.CharField(max_length=30)
    codigo = models.CharField(max_length=2)

    class Meta:
         verbose_name_plural = 'Paises'

    def __unicode__(self):
        return self.nome

class Fabrica(models.Model):
    nome = models.CharField(max_length=30)
    pais = models.ForeignKey('cars.Pais')

    def __unicode__(self):
        return self.nome


class Carro(models.Model):
    nome = models.CharField(max_length=30)
    fabrica = models.ForeignKey("cars.Fabrica")